'98 speedometer

today when i was going to church, i was slowing down from about 70ish mph and when i got to about 45 my speedometer just dropped down to 0. i got to a traffic light about a quarter to half a mile away, stopped, then went and it was working, but now the "service engine soon" light came on. when i went home i was going good till i had to slow down to turn on to my road then it dropped from 45-0 again, but once i got back on the gas it was working again...any ideas?